From time to time, it is necessary to flush water systems which deliver potable tap water. This is especially the case with newly lined fresh water mains which have been repaired by lining interior surfaces of the mains with a resin material. Before water passing through a pipe which is used for drinking water purposes, it is necessary to thoroughly flush the mains with tap water. In order to maintain water quality, it is also necessary, from time to time, to flush local tap water delivery systems such as neighborhood and subdivision tap water systems.
Generally, discharges from potable water systems result from overflow, flushing, disinfection, hydrostatic testing, mechanical cleaning or dewatering of vessels or structures used to store or convey potable water. This frequently includes fire hydrant flushing in which high velocity streams are generated on the order of 2,000 gallons per minute for a period of 10-15 minutes. By periodically testing fire hydrants, it can be determined if sufficient water is available in the system for fire fighting purposes.
Potable tap water usually contains residual chlorine. The Federal Clean Water Act and state agency regulations regarding discharges of potable water, such as regulations promulgated by the Maryland Department of the Environment, require that total maximum daily amount of residual chlorine must be less than 0.1 mg/liter. At levels higher than 0.1 mg/liter, aquatic life is endangered and fish kills occur. Since potable tap water and water discharged from fire hydrants which originates with county and municipal water systems is necessarily initially chlorinated, heavy discharges of this water will adversely affect aquatic life unless the amount of residual chlorine is reduced to less than 0.1 mg/liter. In the past, this was either not done or, when done, was attempted by injecting sodium sulfite into the discharge stream. This is a difficult process to perform and monitor because it is necessary to dispense sodium sulfite in controlled amounts according to the volume of water being treated. If there is too much sodium sulfite, it can itself cause pollution problems by interfering with ph levels and if the amount is insufficient, there will be excessive residual chlorine. It has been found that the injection approach requires not only highly skilled personnel, but extensive training. Moreover, the end result is unpredictable.